--
Example: Community zonation: along gradients of
physical conditions, species gradually replace one another

C. What do ecologists measure in
communities?

1. The abundance
of organisms in a community
2. The diversity
(or number of species) in a community
3. Interactions
between species

II. Food webs: the species in the community and their
interactions

A. Information in food webs

1. Connections
between species in a community

- Food webs with similar numbers of species can
differ in their complexity

2. Energy flow
between species
3. Role of
specific species in controlling populations
4. Keystone
species are species that have have a particularly important impact
on abundances of other species in the food web

B. The role of keystone species

1. Case 1:
Keystone predators: Starfish in rocky intertidal zones

2. Case 2:
Red-naped sapsuckers in aspen forests

C. A related concept is that
of an ecosystem engineer

-- A species that modifies its environment in a way
that substantially affects the community

D. Which are more important in controlling
the structure of communities?